FIG. 11 is a perspective view of a conventional semiconductor packaging device 1 comprising a substrate 4 for mounting a semiconductor device 3, and a package 5 having a space for accommodating the semiconductor device 3 and for supporting the substrate 4. The package 5 is closed downwardly by a lead member 6. A lead wire 21 is electrically connected to the semiconductor device 3.
Recently, the speed of operation and capacity of ICs have been increased and the ICs have come to be used with higher frequencies, so that the ability of such semiconductor devices 3 has been increased The packaging device shown in FIG. 11 can not sufficiently diffuse or discharge the generated heat out of the device. Accordingly, there have been various demands for radiating fins for diffusing the generated heat to the outside of the semiconductor packaging device 1. These heat radiating fins must be light in weight and have a high thermal conductivity.
FIG. 12 is a cross-sectional view of a conventional semiconductor packaging device having a radiating fin 2 attached to the semiconductor packaging device of FIG. 11. The substrate 4 is made of a Mb--W composite material or a Cu--W composite material. The package 5 is made of alumina. The lead member 6 is made of Kovar (29% Ni--17% Co--Fe). The radiating fin 2 is formed of an aluminum alloy which is light in weight. The radiating fin 2 is attached to the rear surface of the substrate 4 by a screw 7 provided on the rear surface of the substrate 4.
FIG. 13 is a cross-sectional view of a conventional compact semiconductor packaging device having a radiating fin similar to that of FIG. 12, except for the following points. Thus, corresponding portions are denoted by the same reference numerals and the description thereof is not repeated.
In order to make the device more compact, wings 2a of the radiating fin 2 are structured to project upwardly. The radiating fin 2 is adhered to the substrate 4 by an adhesive 8 including a filler with good thermal conductivity.
The conventional semiconductor packaging device having the radiating fin structured as described above has the following disadvantages. Namely, when the radiating fin 2 is connected to the substrate 4 by means of a screw 7 as shown in FIG. 12, the area of contact between the radiating fin 2 and the substrate 4 cannot be made sufficiently large, so that heat radiation is insufficient. Further, when the radiating fin 2 and the substrate 4 are connected by an adhesive 8 as shown in FIG. 13, heat radiation is also insufficient since the thermal conductivity of the adhesive 8 is inferior. In the semiconductor packaging device shown in FIG. 13, the thermal expansion coefficient of the substrate 4 made of a Mb--W composite material or a Cu--W composite material, and the thermal expansion coefficient of the package 5 made of alumina are different from that of the radiating fin 2 made of aluminum. As a result, peeling occurs after connection at the boundary during a heat cycle test, generally performed in the range of -50.degree. C. to 150.degree. C., 300 cycles. Such peeling is a problem in actual use.
In order to solve the above described problem, the inventors of the present invention used Mo--Cu composite materials or W--Cu composite materials separately developed for substrates for loading semiconductor devices (Japanese Patent Publication No. 2-31863) for making the radiating fin The thermal expansion coefficient of the Mo--Cu composite materials or of W--Cu composite materials is near that of aluminum or Kovar. Therefore, the above mentioned problem of peeling does not arise during the heat cycle test, when these composite materials are used for the radiating fin. The Mo--Cu composite materials and W--Cu composite materials provide a heat radiating fin having superior thermal characteristics. Especially, the Mo--Cu composite materials having a relatively small specific gravity of 9 to 10 g/cc are practically used at present for making the light radiating fins.
However, even the Mo--Cu composite materials are heavier than aluminum having a specific gravity of 2.7 g/cc which has been used for making the radiating fins. Thus, fins of Mo--Cu are too heavy, especially when they are used for making a large radiating fin recently demanded for large semiconductor chips.
In order to solve this problem, the Mo--Cu composite material is used to make the connecting member 9 for connecting the radiating fin 2 made of aluminum and the package 5 made of alumina, and the connecting member 9 and the radiating fin 2 is connected by a solder 22, lead--tin, gold--tin alloy, and so on, as shown in FIG. 14.
However, referring to FIG. 14, the aluminum, which is the material of the radiating fin 2 is readily oxidized and normally the surface thereof is covered with a thin oxide film. Therefore wetting by the solder is not good, and plating, normally nickel plating after zincate processing, is necessary on the surface of the aluminum to enable soldering. Further, molybdenum existing at the junction surface of the Mo--Cu composite material constituting the connecting member 9, makes the wetting by the solder worse. Accordingly, the junction surface of the Mo--Cu composite material must be plated by nickel plating or the like to enable soldering. In addition, when both are connected by soldering, a certain number of cavities are generated in the soldering layer. Such cavities prevent a good thermal conduction. The thermal conductivity of the solder itself is low, and therefore the solder material also prevents a good thermal conduction.
Referring to FIG. 14, the body formed by the fin 2 and by the connecting member 9 provided by the above described method, must be plated with nickel, nickel+gold, or the like after soldering; in order to provide corrosion resistance. However, plating of the body by various and many materials was difficult, since various materials react differently against chemicals.